Improving the mechanical properties of an Al–Si-coated 22MnB5 steel resistance spot welding joint by adding Ni foil to suppress Fe–Al IMCs and δ-ferrite

Resistance spot welding (RSW) of press-hardened steel (PHS) with Al-Si coatings is challenging due to brittle fracture. In this paper, the joint alloying is achieved through the preset Ni foil welding method, and the microstructure, hardness, and strength of the joint are investigated. The results show that the microstructure of the quenched Al-Si-coated steel RSW nugget tongue consists of Fe-Al IMCs and δ-ferrite. Softening is observed at the boundary between the HAZ and the nugget. Cracks are generated at the interface of the Fe-Al IMCs and the HAZ during the tensile process. The fracture mode is mixed ductile‒brittle fracture. With Ni addition, the microstructure transformation changes to lath martensite (LM) + austenite (γ) + Fe-Ni-Al solid solution + Ni. The brittle Fe-Al IMCs in the solder joints are eliminated. Boundary layer softening is eliminated. The microhardness at the boundary between the nugget and the HAZ increases from 362.2 HV to 441.8 HV due to Ni solid solution strengthening. The fracture mode is ductile fracture, and the fracture force value has increased to 23.33 kN. The mechanical properties of the solder joints are improved.